A thyristor that is known as a power device is formed in a single crystal silicon substrate and kept in a conduction state with a trigger signal such as current (e.g., see Patent Document 1).
A thyristor includes a pnpn layer in which p-type semiconductor layers and n-type semiconductor layers are alternately arranged. An equivalent circuit of a thyristor is composed of an npn bipolar transistor (hereinafter referred to as an npn transistor) and a pnp bipolar transistor (hereinafter referred to as a pnp transistor). FIG. 12 illustrates a specific circuit configuration.
FIG. 12 illustrates a thyristor 1100 including an npn transistor 1101 and a pnp transistor 1102. In the thyristor 1100, an emitter terminal of the npn transistor 1101 is connected to a wiring 1103 to which a high power supply potential VDD is supplied; a collector terminal of the npn transistor 1101 is connected to a base terminal of the pnp transistor 1102 and an input terminal IN to which a trigger signal is supplied; a base terminal of the npn transistor 1101 is connected to a collector terminal of the pnp transistor 1102; and an emitter terminal of the pnp transistor 1102 is connected to a wiring 1104 to which a low power supply potential VSS is supplied.
Operation of the thyristor in FIG. 12 will be briefly described. When a trigger signal is an L signal (also referred to as a low-level signal or a low potential signal), electrical continuity is not established between the collector terminal and the emitter terminal of the pnp transistor 1102 (i.e., the pnp transistor 1102 is turned off), and a current flowing through the collector terminal of the pnp transistor 1102 (hereinafter referred to as a collector current) is hardly detected. Thus, a current flowing through the base terminal of the npn transistor 1101 (hereinafter referred to as a base current) is hardly detected, so that the npn transistor 1101 is also turned off and almost no current flows between the wiring 1103 and the wiring 1104. When a trigger signal is an H signal (also referred to as a high-level signal or a high potential signal), electrical continuity is established between the collector terminal and the emitter terminal of the pnp transistor 1102 (i.e., the pnp transistor 1102 is turned on), and a collector current is detected at the collector terminal of the pnp transistor 1102. Thus, a base current of the npn transistor 1101 flows, and the npn transistor 1101 is brought into conduction. When the npn transistor 1101 is brought into conduction, a collector current of the npn transistor 1101 is detected, and the pnp transistor 1102 is kept in a conduction state. The thyristor 1100 has a feature such that a large current obtained by adding the collector current of the pnp transistor 1102 to the collector current of the npn transistor 1101 flows between the wiring 1103 and the wiring 1104.